Method and apparatus for reducing bubbles or gas pockets in a metal ingot using a continuous casting mold

ABSTRACT

A method and apparatus are provided to cast a metal ingot using a continuous casting mold so that the ingot is essentially free of gas pockets which otherwise would result from gas bubbles being entrapped in the mushy zone and solid portion of the ingot during formation, wherein such bubbles may be caused by pouring molten metal into a molten liquid portion of the forming ingot and by impingement of a plasma plume of a plasma torch on the upper surface of the molten liquid portion.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates generally to furnaces for melting metals. More particularly, the invention relates to a continuous casting furnace and method for eliminating or reducing gas bubbles or pockets within an ingot.

2. Background Information

One of the problems in the continuous casting of metal ingots is the formation or entrapment of bubbles or gas pockets in the ingot. As discussed in greater detail further below, such bubbles or gas pockets may be caused by the pouring of molten material into the molten metal within the continuous casting mold or by the impingement of the plume of a plasma torch along the top surface of molten material in the mold.

SUMMARY

In one aspect, the invention may provide a method comprising the steps of: providing a segmented continuous casting mold comprising in alternating fashion a plurality of electrically conductive upwardly extending fingers and a plurality of electrically nonconductive high temperature insulation pieces; withdrawing a metal ingot from the mold wherein the ingot comprises a solid portion, a mushy zone on top of the solid portion and a liquid portion on top of the mushy zone; producing gas bubbles in the liquid portion by at least one of (a) pouring molten metal into the liquid portion and (b) heating the liquid portion with a plasma plume of a plasma torch; and heating the liquid portion with an induction coil adjacent the mold whereby the liquid portion comprises a liquid metal head and a bubble zone such that the liquid metal head is on top of the mushy zone and essentially free of gas bubbles, and the bubble zone is on top of the liquid metal head and comprises gas bubbles.

In another aspect, the invention may provide a method comprising the steps of: providing a segmented continuous casting mold comprising in alternating fashion a plurality of electrically conductive upwardly extending fingers having respective tops and a plurality of electrically non-conductive high temperature insulation pieces; withdrawing a metal ingot from the mold; pouring molten metal into the mold atop the ingot; preventing the pouring molten metal from contacting the tops of the fingers with a top member disposed directly above and adjacent the fingers; and heating a portion of the ingot within the mold with an induction coil adjacent the mold.

In another aspect, the invention may provide an apparatus comprising: a segmented continuous casting mold comprising in alternating fashion a plurality of electrically conductive upwardly extending fingers having respective tops and a plurality of electrically non-conductive high temperature insulation pieces; a top member disposed directly above and adjacent the tops of the fingers and adapted to prevent pouring molten metal from contacting the tops of the fingers; and an induction coil adjacent the mold.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Preferred embodiments of the invention, illustrative of the best mode in which Applicant contemplates applying the principles, are set forth in the following description and are shown in the drawings and are particularly and distinctly pointed out and set forth in the appended claims.

FIG. 1 is a prior art diagrammatic sectional view of a continuous casting apparatus showing bubbles or gas pockets trapped in an ingot.

FIG. 2 is a diagrammatic view of a sample embodiment as viewed from the side with various components shown in section.

FIG. 3 is a top plan view taken on line 3-3 of FIG. 2 looking down on the top member and portions of the continuous casting mold.

FIG. 4 is a sectional view taken on line 4-4 of FIG. 3 of the continuous casting mold.

FIG. 5 is a sectional view taken on line 5-5 of FIG. 4.

FIG. 6 is an operational view taken from the same perspective as FIG. 2 and showing the formation of an ingot essentially without the entrapment of gas pockets.

Similar numbers refer to similar parts throughout the drawings.

DETAILED DESCRIPTION

With reference to FIG. 1, the prior art problem is described in greater detail. A prior art continuous casting furnace may include a continuous casting mold CCM, a melting hearth H and a plasma torch T above the mold. During formation of the ingot I, the ingot will have a solid zone or portion S, a mushy zone M above the solid portion S, and a liquid portion or pool L above the mushy zone M. As is well known in the art, the mushy zone M is a transitional zone which is partially liquid and partially solid and which becomes solid as the ingot I is lowered and cooled. When molten metal MM is poured from hearth H into the mold CCM atop the liquid portion L, the pouring molten metal forces some gas bubbles BP, which may be inert gas bubbles, into the liquid zone L. When a plasma torch is used, the plasma plume P thereof impinges on the upper surface of the liquid zone L and may also create bubbles BT in the liquid zone L. As ingot I cools and is lowered, some of the bubbles BP and BT may also enter the mushy zone M and ultimately be trapped in the solid portion S such that ingot I is formed with gas bubbles or gas pockets BP, BT. Unfortunately, these gas pockets or porosities cannot be removed by thermo-mechanical processing of the ingot and are detrimental to mechanical properties of the final product.

A sample embodiment of the present furnace is shown generally at 1 in FIG. 2. Furnace 1 may include a melting chamber 2, a segmented continuous casting mold 4, a melting hearth 6, a feed mechanism 8, a hearth heat source such as a hearth plasma torch 10, a mold heat source such as a mold plasma torch 12, an induction coil 14, a cover or top plate or member 16, an ingot lift 18, a vacuum pump 20 and an inert gas source 22. Melting chamber 2 includes a chamber wall 24 which defines an interior chamber 26 in which are disposed mold 4, hearth 6, feed mechanism 8, torches 10 and 12, induction coil 14 and top member 16. The top of ingot lift 18 is moveable into and out of interior chamber 26. More particularly, melting chamber 2 further includes an annular passage wall 28 which is secured to wall 24 and has an inner perimeter which defines a passage 30 which communicates with interior chamber 26 and atmospheric external to wall 24 and chamber 2. The top of ingot lift 18 is thus moveable into and out of interior chamber 26 via passage 30. Furnace 1 may be supplied with various types of sealing assemblies in the area of passage wall 28, such as those described in U.S. Pat. No. 8,196,641, which is incorporated herein by reference. Generally, these sealing assemblies form a seal with the outer perimeter of an ingot to prevent air from entering chamber 26 via passage 30 during ingot casting.

Feed mechanism 8 may be any type of feed mechanism, such as a conveyor, a vibratory feeder or other feed mechanism known in the art for feeding solid metal into a melting cavity 32 of hearth 6. Hearth 6 may further include an overflow or pouring lip 34 along one side to allow molten metal to move from cavity 32 through overflow or lip 34 into the top of mold 4. Hearth torch 10 is positioned above melting cavity 32 in order to heat metal within cavity 32, while mold torch 12 is likewise positioned above mold 4 in order to heat metal within mold 4. Induction coil 4 may circumscribe mold 4, as described in greater detail further below. Top member 16 is an annular member which is positioned directly above and adjacent the top of mold 4.

With primary reference to FIGS. 3-5, mold 4 includes a mold side wall 36 having an inner perimeter 38 which defines a mold cavity or passage 40 having a top entrance opening 42 and a bottom entrance opening 44. Passage 40 has a central vertical axis X which lies at the center of passage 40. Inner perimeter 38 has a substantially vertical lower portion which defines a narrower lower cavity portion 46 of cavity or passage 40, and a substantially vertical upper portion which defines a wider upper cavity portion 48 of cavity or passage 40. When lower and upper portions 46 and 48 are cylindrical (typically with inner perimeter 38 concentric about axis X), the inner diameter of narrower portion 46 is thus less than the inner diameter of wider portion 48. Inner perimeter 38 steps inwardly at a step 50 from the bottom of upper portion 48 to the top of lower portion 46. Step 50 may angle radially inwardly and downwardly toward axis X from the bottom of upper portion 48 to the top of lower portion 46. Side wall 36 may include an electrically conductive or metal portion 52 having a lower section 54 and an upper section 56, each of which is annular. Lower section 54 defines an annular cavity 58 which surrounds or circumscribes the lower portion of lower cavity portion 46 adjacent bottom entrance opening 44. Metal portion 52, which may be formed of copper, includes upwardly extending fingers 60 which are cantilevered upwardly from the top of lower section 54 to terminal tops or top ends 62 which define the top of side wall 36 and mold 4.

Each finger 60 thus has a bottom 64 such that the bottoms 64 serve as the bottom of upper section 56 and the top of lower section 54. Each finger 64 is thus rigidly secured at its bottom 64 to the top of lower section 54 and extends upwardly or is cantilevered upwardly therefrom. Each finger 60 may be substantially vertical and include an inner surface 66, an outer surface 68, and a pair of opposed side surfaces 70 and 72 which extend radially outwardly from axis X from inner surface 66 to outer surface 68. Inner surfaces 66 of fingers 60 form the great majority of wider upper cavity portion 48 and an upper portion of narrower lower cavity portion 46. For each adjacent pair of fingers 60, the side surface 70 of one of the pair and the side surface 72 of the other finger of the pair are typically vertical, parallel and closely adjacent one another and define therebetween a slot or gap 74 extending from bottom 64 to top 62 and from inner surface 66 to outer surface 68.

Side wall 36 further includes a refractory insulation piece 76 within each gap 74 extending from adjacent bottom 64 to adjacent top 62 such that opposed sides of each piece 76 respectively abut the side surface 70 and side surface 72 which defines a given gap 74. The inner surface of each piece 76 may be substantially flush with the inner surfaces 66 of the adjacent pair of fingers which define the gap in which piece 76 is disposed. Likewise, the outer surface of each piece 76 may be generally flush with the outer surface 68 of the pair of fingers 60 defining the gap 74 in which the given piece 76 is disposed. Each insulation piece 76 may be formed of mica or any other suitable high temperature insulation material which is electrically non-conductive. Fingers 60 and pieces 76 have upper segments which define upper cavity portion 48 and lower segments which define an upper part of lower cavity portion 46. Each pair of surfaces 70 and 72 which define a given gap are not directly in electrical contact with one another although all of fingers 60 are in electrical communication with one another via their physical and electrical contact with lower section 54 of metal portion 52. Thus, mold 4 includes in alternating fashioning a plurality of electrically conductive upwardly extending fingers 60 and a plurality of upwardly extending electrically non-conductive high temperature insulation pieces 76. The sample mold 4 includes 16 fingers 60 although this number may vary depending on the specific desired characteristics. The top of induction coil 14 may be adjacent the tops 62 of fingers 60, the tops of pieces 76 and the top of mold 4, while the bottom of induction coil 14 may be adjacent the bottoms 64 of fingers 60 and the bottoms of pieces 76.

A plurality of substantially vertical finger passages 78 are formed in metal portion 52 each having a closed top end 80 and a bottom entrance opening 82 which communicates with annular cavity 58. Each finger passage 78 extends upwardly into a given finger 60 and thus extends from the bottom of the given finger 60 to adjacent and below the top 62 thereof. A hollow ring 84 is disposed in annular cavity 58 and thereby circumscribes a lower portion of lower cavity portion 46. Ring 84 defines an annular passage 85. A plurality of fluid tubes 86 are connected to and extend upwardly from the top of ring 84 and respectively into finger passages 78. Each fluid tube 86 may be substantially vertical and define a substantially vertically passage 88 having a top entrance opening 90 adjacent and below closed top end 80 and a bottom entrance opening 92 which communicates with annular passage 85. Lower section 54 defines a fluid or water inlet 94 which is in fluid communication with annular cavity 85, and a fluid or water outlet 96 which is in fluid communication with annular cavity 58. Fluid feed and return lines 98 and 100 are respectively connected to inlet 94 and outlet 96 at one end and at another end to a water source or other cooling fluid source 102.

Top member 16 (FIGS. 2, 3, 6) includes a generally hollow annular wall 104 which thus defines an annular pouring passage 106. Wall 104 has an annular circular upwardly facing generally horizontal top surface 108 and an annular circular downwardly facing generally horizontal bottom surface 110 which is adjacent and spaced upwardly from top ends 62 of fingers 60 and the top of mold 4. Wall 104 has an annular circular inner surface or perimeter 112 which faces radially inwardly and extends from top surface 108 to bottom surface 110 and an annular circular outer surface or perimeter 114 which faces radially outwardly and extends from top surface 108 to bottom surface 110. Inner perimeter 112 defines a through passage 116 having a top entrance opening 118 and a bottom entrance opening 120 which is directly above top entrance opening 42 such that passage 116 and passage 40 are in fluid communication with one another. Inner perimeter 112 may be directly above or radially inward of the upper portion of inner perimeter 38 which defines upper cavity portion 48, whereas outer perimeter 114 may be directly above or radially outward of the outer perimeter of wall 104/outer surfaces 68 of fingers 60 so that top member 16 entirely covers tops 62 of fingers 60 so that tops 62 are not visible as viewed from above, as shown in FIG. 3.

Wall 104, which is formed of an electrically conductive material such as copper or another metal, forms a nearly continuous ring except adjacent circumferential ends 122 and 124 thereof (FIG. 3) which face one another and are closely adjacent one another to define therebetween a gap 126 which extends continuously from top surface 108 to bottom surface 110 and from inner perimeter 112 to outer perimeter 114. In the sample embodiment, gap 126 is filled with grout 128 or another electrically non-conductive material. As shown in FIG. 3, wall 104 along outer perimeter 114 defines a water or liquid inlet 130 adjacent end 122 and a liquid or water outlet 132 adjacent end 124 so that inlet 130 and outlet 132 are on opposite sides of and adjacent gap 126 and grout 128. A feed line 134 is connected at one end to inlet 130 and at an opposite end to cooling fluid source 102 or another like source. A return line 136 is likewise connected at one end to outlet 132 and at the opposite end to source 102 or another like source. Although not shown in the Figures, induction coil 14 is also water cooled or liquid cooled and thus typically has a feed line extending between an inlet of induction coil 14 to a source such as source 102 and a return line extending from an outlet of induction coil 14 to a source such as source 102.

The operation of the sample furnace is now described with primary reference to FIG. 6. Where an inert gas atmosphere is desired within chamber 26, such as is the case with the use of plasma torches, vacuum pump 20 is used to evacuate chamber 26 and thus substantially remove all air therefrom, then back filled with inert gas from source 22 to produce an inert gas atmosphere within chamber 26 which is all or substantially all inert gas such as argon or helium. Solid metal is moved into cavity 32 of hearth 6 from feed mechanism 8 (FIG. 2) and hearth torch 10 is ignited to heat and melt the solid material to form molten material MM in cavity 32 and to keep heating it to maintain it in molten form.

Molten metal 2 is then poured through passage 116 of top member 16 into passage 40 of mold 4 on top of the top of lift 18 or on a starter stub within mold 4. Top member 16 is configured to prevent molten metal which is being poured from hearth 6 into mold 4 from contacting the top 62 of fingers 60 and the top of mold 4, including the top of the insulation pieces 76. Induction coil 14 is electrically powered in order to inductively heat the molten metal within mold 4 while water or another cooling liquid is moved from source 102 through the induction coil 14, the water cooling system of mold 4 and top member 16 to cool induction coil 14, mold 4 and top member 16. More particularly and with reference to FIG. 4, the cooling liquid flows (various arrows in FIG. 4) through feed line 98 into annular cavity 85 and from cavity 85 upwardly into passages 88 via entrance openings 92, and from passages 88 into finger passages 78 via exit openings 90. The liquid then flows downwardly from the top of passages 78 to the bottom of passages 78 into cavity 58 via bottom entrance openings 82, and from cavity 58 into return line 100 back to source 102. As illustrated by the arrows in FIG. 3, water or another cooling liquid is likewise pumped through feed line 134 from source 102 into annular passage 106 of top member 16 via inlet 130, circulated through passage 106 to outlet 132, and from outlet 132 back to source 102 via return line 136. Mold torch 12 is also ignited to produce plasma plume P in order to heat the molten metal of liquid portion L within mold 4 from the top.

As previously described with reference to the prior art example of FIG. 1, the pouring of molten material MM (FIG. 6) atop the liquid portion or zone L produces bubbles BP within liquid portion L, with such bubbles being typically formed of an inert gas such as argon or helium, especially in the context of melting with plasma torches. In addition, where the melting involves the use of mold plasma torch 12, plume P of the torch impinges on the upper surface of the liquid portion L and likewise forms bubbles BT therein as plume P heats liquid portion L. However, unlike the prior art systems, the sample embodiment illustrates that the liquid zone L may be divided into a bubble zone BZ which lies along the upper portion of liquid portion L and includes bubbles BP and/or BT, and a liquid metal head MH directly below bubble zone BZ which is free of or essentially free of bubbles BP or BT. Metal head MH is directly above and in contact with the top of mushy zone M, which is directly above and in contact with the top of solid portion S of ingot I. The use of induction coil 14 and mold 4 allows the molten pool or liquid portion L to have a greater depth than previously known, so that bubbles BP and BT are allowed sufficient time to move upwardly to the upper surface of liquid portion L before becoming entrapped in the mushy zone M and solid portion 5, as was the case in the prior art systems and methods. As a result, mushy zone M and solid portion S are free of or essentially free of bubbles or gas pockets. The molten or liquid portion L extends from above step 50 to below step 50 and generally to or below the bottom 64 of fingers 60 and the bottom of coil 14. As ingot I is lowered or withdrawn from mold 4 via lift 18 and additional molten metal MM is poured atop the liquid portion L in mold 4, the lower portion of liquid portion L or metal head MH cools and becomes a part of mushy zone M and subsequently a portion of solid portion S of the ingot, while metal head MH is maintained free of the bubbles BP and BT throughout the casting process so that ingot I is formed essentially free of gas pockets.

The sample method and apparatus thus provide a way to produce ingots with a continuous casting mold wherein the ingots are free of or essentially free of gas pockets formed therein as a result of bubbles caused by the pouring of molten metal into the mold and/or the bubbles formed by the plasma plume of a plasma torch impinging on the upper surface of the molten metal within the mold. The sample method and apparatus thus provide the ability to provide substantially improved ingots.

In the foregoing description, certain terms have been used for brevity, clearness, and understanding. No unnecessary limitations are to be implied therefrom beyond the requirement of the prior art because such terms are used for descriptive purposes and are intended to be broadly construed. Moreover, the description and illustration of the preferred embodiment of the invention are an example and the invention is not limited to the exact details shown or described. 

1. A method comprising the steps of: providing a segmented continuous casting mold comprising in alternating fashion a plurality of electrically conductive upwardly extending fingers and a plurality of electrically nonconductive high temperature insulation pieces; withdrawing a metal ingot from the mold wherein the ingot comprises a solid portion, a mushy zone on top of the solid portion and a liquid portion on top of the mushy zone; producing gas bubbles in the liquid portion by at least one of (a) pouring molten metal into the liquid portion and (b) heating the liquid portion with a plasma plume of a plasma torch; and heating the liquid portion with an induction coil adjacent the mold whereby the liquid portion comprises a liquid metal head and a bubble zone such that the liquid metal head is on top of the mushy zone and essentially free of gas bubbles, and the bubble zone is on top of the liquid metal head and comprises gas bubbles.
 2. The method of claim 1 further comprising the step of heating the liquid portion with a plasma plume of a plasma torch.
 3. The method of claim 1 further comprising the step of pouring molten metal into the liquid portion.
 4. The method of claim 3 wherein the step of pouring comprises pouring molten metal into the liquid portion from a hearth; wherein the hearth and mold are within an inert gas atmosphere.
 5. The method of claim 4 further comprising the step of heating the liquid portion with a plasma plume of a plasma torch.
 6. The method of claim 3 further comprising the step of preventing the pouring molten metal from contacting the tops of the fingers with a top member disposed directly above and adjacent the fingers.
 7. The method of claim 1 further comprising the step of pouring molten material into the mold through a pouring passage defined by an annular top member disposed directly above and adjacent the fingers.
 8. The method of claim 7 wherein the top member has a top surface, a bottom surface, an outer perimeter, and an inner perimeter which defines the pouring passage; and a gap is formed in the top member extending from the top surface to the bottom surface and from the outer perimeter to the inner perimeter.
 9. The method of claim 8 further comprising an electrically non-conductive material in the gap.
 10. The method of claim 7 further comprising the step of cooling the top member by moving a cooling liquid through a cooling passage formed in the top member.
 11. The method of claim 7 wherein the top member entirely covers the tops of the fingers so that the tops of the fingers are not visible as viewed from above.
 12. The method of claim 1 wherein the mold has a sidewall having an inner perimeter which defines a mold cavity in which the liquid portion is disposed; the inner perimeter defines a wider upper cavity portion and a narrower lower cavity portion and includes a step which extends inwardly from the wider upper cavity portion to the narrower lower cavity portion.
 13. The method of claim 12 wherein the step angles inwardly and downwardly from the wider upper cavity portion to the narrower lower cavity portion.
 14. The method of claim 12 wherein the fingers comprise respective upper segments which define the wider upper cavity portion and respective lower segments which define at least a portion the narrower lower cavity portion.
 15. The method of claim 14 wherein the sidewall has a lower section; and the fingers are cantilevered upwardly from the lower section.
 16. The method of claim 1 wherein the induction coil circumscribes the mold.
 17. The method of claim 16 wherein the fingers have respective bottoms; and the induction coil has a bottom adjacent the bottoms of the fingers.
 18. The method of claim 17 wherein the fingers have respective tops; and the induction coil has a top adjacent the tops of the fingers.
 19. A method comprising the steps of: providing a segmented continuous casting mold comprising in alternating fashion a plurality of electrically conductive upwardly extending fingers having respective tops and a plurality of electrically non-conductive high temperature insulation pieces; withdrawing a metal ingot from the mold; pouring molten metal into the mold atop the ingot; preventing the pouring molten metal from contacting the tops of the fingers with a top member disposed directly above and adjacent the fingers; and heating a portion of the ingot within the mold with an induction coil adjacent the mold.
 20. An apparatus comprising: a segmented continuous casting mold comprising in alternating fashion a plurality of electrically conductive upwardly extending fingers having respective tops and a plurality of electrically non-conductive high temperature insulation pieces; a top member disposed directly above and adjacent the tops of the fingers and adapted to prevent pouring molten metal from contacting the tops of the fingers; and an induction coil adjacent the mold. 